In photolithographic processes for manufacturing semiconductor devices and the like, a projection exposure apparatus is used that exposes the pattern image of a photomask or reticle (hereinafter, collectively referred to as "reticle") as the projection master plate onto a substrate (wafer or glass plate and the like) coated with a photosensitive material like photoresist via a projection optical system.
With the increase in the level of integration of semiconductor devices and the like in recent years, the resolving power required by projection optical systems used in projection exposure apparatuses has been rapidly increasing. To meet this requirement, it has become necessary to shorten the wavelength of the exposure light and to increase the numerical aperture (NA) of the projection optical system. However, if the wavelength of the illumination light is shortened, particularly below 300 nm, the number of types of glass materials that can be used for practical purposes is limited to a few due to the absorption of light. Accordingly, the correction of chromatic aberration becomes problematic if the projection optical system is constructed with just dioptric optical elements. In addition, a dioptric optical system requires numerous lenses to correct the Petzval sum.
In contrast, a catoptric system not only has no chromatic aberration, but can also easily correct the Petzval sum by the use of a concave reflecting mirror. Accordingly, the construction of projection optical systems with so-called catadioptric optical systems that combine a catoptric system with a dioptric system has heretofore been proposed. Such catadioptric optical systems have been proposed in, for example, U.S. Pat. No. 5,537,260, U.S. Pat. No. 4,747,678, U.S. Pat. No. 5,052,763 and U.S. Pat. No. 4,779,966.
An increase in the NA and exposure region of projection optical systems has been demanded in recent years, and the aperture diameter of the optical members that constitute catadioptric optical systems has likewise increased. In light of the resolution required by projection exposure apparatuses, the effect of deformation of large optical members due to gravity cannot be ignored. A catadioptric optical system may be constructed with optical members in which the direction of the optical axes is not identical, as is typical of the prior art. One or more such optical members having power tend to be deformed asymmetrically with respect to the optical axis. This gives rise to asymmetric aberrations, which are difficult to correct during manufacturing, making it difficult to obtain sufficient resolution.
When correcting chromatic aberration in a conventional catadioptric optical system, high-order chromatic aberrations like transverse chromatic aberration, cannot be sufficiently corrected with just a concave reflecting mirror and quartz glass, and the image size cannot be increased. Consequently, attempts are being made to realize satisfactory correction of chromatic aberration over the entire exposure region by forming a number of lenses with fluorite. Nevertheless, since the volume and refractive index of lenses made of fluorite change much more than quartz glass and other optical glasses when environmental factors, like temperature, change, the optical performance of conventional optical systems deteriorates greatly when the environmental conditions fluctuate.
Catadioptric optical systems and catoptric optical systems typically require the use of a folding member to separate the optical path of the going path toward the concave mirror from the optical path of the returning path from the concave mirror. As a result, a plurality of partial optical systems having mutually different optical axes becomes necessary, and it follows that a plurality of lens barrels having different axes becomes necessary. Consequently, there is the problem that, compared with dioptric optical systems, errors are easily generated in the adjustment between the plurality of optical axes when assembling catadioptric optical systems and catoptric optical systems. In addition, even after assembly, the stability is poor due to the complex construction, and the positional relationships between optical axes gradually deviate, creating a tendency for the image to deteriorate. In addition, the folding member has an incident optical axis and an exit optical axis, which are not formed symmetrically. For this reason, rotating the folding member about the incident optical axis or about the exit optical axis due to, for example, factors like vibration, causes rotation of the image. In addition, rotating the folding member about the axis orthogonal to both the incident optical axis and the exit optical axis causes distortion of the image, making it difficult to stably obtain an image of high resolution.
Furthermore, optical adjustment of a dioptric optical system is disclosed in U.S. Pat. No. 4,711,567 and Japanese Pat. Application Kokai No. Hei 10-54932, and optical adjustment of a catadioptric optical system is disclosed in U.S. Pat. No. 5,638,223.